Specific Absorption Rate (SAR) Induced in Human
Heads of Various Sizes When Using a Mobile Phone
Adel Z. El Dein Alaeddin Amr
Abstract - This paper analyzes the specific absorption rate
(SAR) induced in human head model of various sizes by a mo-bile phone at 900 and 1800 MHz. Specifically the study is con-sidering in SAR between adults and children. Moreover, these differences are assessed for compliance with international safe-ty guidelines. Also the effects of these head models on the most important terms for a mobile terminal antenna designer, namely: radiation efficiency, total efficiency and directivity, are investigated.
Index Terms—Human Head, Mobile Phone, SAR.
I. INTRODUCTION
In recent years, much attention has been paid to health implication of electromagnetic (EM) waves, especially hu-man head part, which is exposed to the EM fields radiated from handsets. With the recent explosive increase of the use of mobile communication handsets, especially the number of children using a mobile phone, that develops many ques-tions about the nature and degree of absorption of EM waves by this category of public as a function of their age and their morphology. For this reason the World Health Organization (WHO) has recommended to undertake re-search studies on this subject [1]. This paper investigates the effects of head models of various sizes on the most impor-tant terms for a mobile terminal antenna designer, namely: radiation efficiency, total efficiency and directivity; and also on the Specific Absorption Rates (SAR) which are induced in them. For this purpose, a comparison is performed con-cerning those parameters between an adult human head and some children heads obtained as a percent of an adult human head. The results are obtained using an electromagnetic field solver employing the Integral Equations method [2]. The
SAR is the most appropriate metric for determining EM effect exposure in the very near field of a Radio Frequency (RF) source [3-7]. The local SAR (W/kg) at any point in the human head is defined as:
2 2 E SAR (1)
where E is the peak amplitude of the electrical field in the human head tissue (V/m), σ is the tissue conductivity (S/m) and ρ is the tissue density (kg/m3). The SAR over a mass of
10g and 1g in the head and the other parameters of the mo-bile antenna are determined in each case.
II. MODELING OF HUMAN HEAD
For this study, five head models are used namely: that of an adult and other children human heads of sizes; 95%, 90%, 85%, and 80% of the adult head size (which of size 100%), as they are shown in fig. (1). Each head model consists of shell of skin tissue which is filled with a liquid of brain properties. For simulation of the EM fields in the human head, the appropriate parameters for the conductivity σ (S/m), the relative permittivity r and the tissue density ρ
(kg/m3) of all different materials used for the calculation
must be known. Additionally, the frequency dependence of these parameters must be considered and chosen appro-priately. A recent compilation of Gabriel et al. covers a wide range of different body tissues and offers equations to de-termine the appropriate dielectric values at each desired frequency [8].
100% 95% 90% 85% 80% Fig. 1 The description of the various sizes human head models
Table I shows the real part of the dielectric permittivity εr,
conductivity σ (S/m), and mass density ρ (kg/m3) of tissues
used in the simulations at 900 and 1800MHz. Table II
Manuscript received February 24, 2010. Paper no. ICWN_14. Adel Z. El Dein is with the Department of Electrical Engineering, High Institute of Energy, South Valley University, Aswan 81258, Egypt (e-mail:[email protected]).
Alaeddin Amr is with faculty of Computer Engineering and IT Al-Hussein Bin Talal University
shows the volume and the mass of the tissue of all children heads.
Table I
THE DIELECTRIC PERMITTIVITY εr, CONDUCTIVITY σ (S/m), AND MASS DEN-SITY ρ (kg/m3) OF TISSUES USED IN THE SIMULATIONS AT 900 AND
1800MHZ. Properties of Tissues Dielectric
Permittivity εr Conductivity σ (S/m) Mass Density ρ(kg/m3) Shell (Skin) 900MHz 43.8 0.86 1000 1800MHz 38.87 1.19 1000 Liquid (Brain) 900MHz 45.8 0.77 1030 1800MHz 43.5 1.15 1030 Table II
VOLUME AND MASS OF THE HEADS’ MODELS
The Volume and mass of the human Head
Human Head Size as a percent from an adult one. 100% 95% 90% 85% 80% Tissue Volume
(mm3)*106 5.5893 4.7886 4.0706 3.4283 2.8573
Tissue Mass (kg) 5.7439 4.9236 4.1855 3.5250 2.9379
III. MODELING OF THE MOBILE PHONE
The mobile handset consists of a quarter-wavelength mo-nopole (of radius 0.0025m at 900MHz and 0.001m at 1800MHz) mounted on a mobile handset (treated as a metal box of 1.8cm4cm10cm), operates at 900 and 1800MHz and radiated power of 0.125 watt, as it is shown in fig. 2.
Fig. 2 The description of the mobile handset
IV. RESULTS AND DISCUSSION
Figures (3) and (4) present mobile terminal antenna de-signer parameters namely: return loss, radiation efficiency, total efficiency and directivity, the results obtained with the absence of the human head and at a frequency 1800MHz. Table III present the Mobile Antenna Parameters, namely: radiation efficiency, total efficiency and directivity, the re-sults obtained for various sizes of human heads and for the case of there absence. It is seen that as the size of human head decreases the radiation efficiency and total efficiency
Fig. 3 The return loss without human head
Fig. 4 The Far Field without human head Table III
MOBILE ANTENNA PARAMETERS WITH VARIOUS SIZES OF HUMAN HEAD
Mobile Antenna Parameters
Without human Head
Human Head Size as a percent from an adult one. 100% 95% 90% 85% 80% 900 MHz Rad. Tot. 1.003 0.788 0.276 0.292 0.311 0.335 0.357 0.271 0.286 0.305 0.328 0.35 Dir. (dBi) 2.627 6.066 5.943 5.859 5.819 5.712 1800 MHz Rad. 1.003 0.485 0.498 0.512 0.528 0.546 Tot. 1.002 0.476 0.489 0.504 0.521 0.539 Dir. (dBi) 3.653 7.98 7.855 7.756 7.673 7.552
The "SAR 10 grams" is the maximum SAR value aver-aged on 10g which is obtained by averaging the SAR around each point in the volume and adding the nearest points till an average mass of 10g is reached with a resulting volume having the shape of a portion of sphere. The "contiguous SAR 1 gram" is estimated by averaging the local maximum SAR, adding the highest SAR volume in a given tissue till a
heads decrease, as indicated in Table III. Also from Table III it is noticed that, the total SAR over the whole human head at 1800MHz is less than that at 900MHz. This is be-cause the SAR regions produced by monopole antenna at 900 MHz are more extended as compared to those induced at 1800 MHz. The human body works as a barrier, mainly in high frequencies, because of skin depth. As the frequency increases the penetration capacity decreases and become more susceptible to obstacles.
Figures (5-10) show the distributions of the local SAR, at the y=0 plane; 10 grams SAR in xz plane; and 1gram SAR in xy plane; in (W/kg), on the human head of various sizes, obtained with a radiated power of 125mW from a monopole antenna operates at 900 and 1800MHz respectively. It can be easily noticed that high SAR regions produced by 900MHz monopole antenna are more extended as compared
to those induced by 1800 MHz monopole antenna, as it is explained before.
Table IV
SAR INDUCED IN CHILDREN’S HEADS
Calculated Parameters of the
human Head Human Head Size as a percent from an adult one.
100% 95% 90% 85% 80% 900MHz SAR (point) 1.134 1.206 1.124 1.122 1.214 SAR 1g 0.818 0.805 0.785 0.769 0.769 SAR 10g 0.593 0.59 0.584 0.58 0.572 Absorbed Power (wrms) 0.089 0.087 0.085 0.082 0.079 Total SAR (W/kg) 0.016 0.018 0.02 0.023 0.027 1800MHz SAR (point) 4.149 3.078 2.404 2.319 2.282 SAR 1g 1.590 1.530 1.482 1.399 1.312 SAR 10g 0.922 0.887 0.848 0.805 0.764 Absorbed Power (wrms) 0.064 0.062 0.060 0.058 0.056 Total SAR (W/kg) 0.011 0.012 0.014 0.016 0.019 100% 95% 90% 85% 80%
Fig. 5 The distributions of the local SAR at x=0 plane for 1800MHz
Fig. 7. The distributions of the (1g) SAR at xy plane for 1800MHz
Fig. 10 The distributions of the (1g) SAR at xy plane for 900MHz
V. CONCLUSION
The obtained results show that the spatial-peak SAR val-ues at a point or as averaged over 1 gram and 10 grams on the human head of various sizes, obtained with a radiated power of 125mW from a monopole antenna operates at 900 and 1800MHz, vary with the size of the human’s head at each frequency. Also the sizes of the head have an effect on the mobile terminal antenna designer parameters, and this effect can't be eliminated, because it is an electromagnetic characteristic. The obtained results show that the spatial-peak SAR values as averaged over 1 gram on the human head obtained with a radiated power of 0.125 watt for all simulations are well below the limit of 1.6 W/kg, which is recommended by FCC and ICNIRP [9-11].
REFERENCES
[1] R Kitchen,” RF and Microwave Radiation Safety Handbook” Second Edition, Book, Chapter 3, pp.47-85, 2001.
[2] http://www.cst.com/
[3] K. Kiminami, T. Iyama, T. Onishi and S. Uebayashi “Novel Specific Absorption Rate (SAR) Estimation Method Based on 2-D Scanned Electric Fields” IEEE Transactions on Electromagnetic Compatibility, Vol. 50, No. 4, November 2008, pp. 828-836.
[4] S. Watanabe, M. Taki, T. Nojima and O. Fujiwara “Characteristics of the SAR Distributions in a Head Exposed to Electromagnetic Fields
Radiated by a Hand-Held Portable Radio” IEEE Transactions on Mi-crowave Theory and Techniques, Vol. 44. No. 10, October 1996, pp. 1874- 1883
[5] A. Hadjem, D. Lautru, C. Dale, M. F. Wong, V. Fouad-Hanna, J. Wiart, "Comparison of Specific Absorption Rate (SAR) Induced in Child-Sized and Adult Heads Using a Dual Band Mobile Phone" pro-ceeding on IEEE MTT-S Int. Microwave Symp. Dig. June. 2004. [6] O. Kivekäs, J. Ollikainen, T. Lehtiniemi, and P. Vainikainen
“Band-width, SAR, and Efficiency of Internal Mobile Phone Antennas” IEEE Transactions on Electromagnetic Compatibility, Vol. 46, No. 1, February 2004, pp. 71-86
[7] Brian B. Beard and all “Comparisons of Computed Mobile Phone Induced SAR in the SAM Phantom to That in Anatomically Correct Models of the Human Head” IEEE Transactions on Electromagnetic Compatibility, Vol. 48, No. 2, May 2006 pp. 397-407
[8] C. Gabriel (1996). “Compilation of the Dielectric Properties of Body Tissues at RF and Microwave Frequencies” "Brooks Air" Force Technical Report AL/OE-TR-1996-0037 [Online]. Available: http://www.fcc.gov/cgi-bin/dielec.sh
[9] FCC, OET Bulletin 65, “Evaluating Compliance with FCC Guide-lines for Human Exposure to Radiofrequency Electromagnetic Fields,” Edition 97- 01, released December, 1997.
[10] IEEE C95.1-1991, IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz (Institute of Electrical and Electronics Engineers, Inc., New York, 1992).
[11] European Committee for Electrotechnical Standardization (CENE-LEC) Prestandard ENV 501 66- 2, Human exposure to electromag-netic fields. High frequency (10 kHz to 300 GHz) (January 1995).